The invention relates to a process for producing pig iron, in which the iron ore is reduced in a reducing shaft furnace and the resulting sponge iron is subsequently melted in a melting gasifier into which are introduced a carbon carrier and an oxidizing gas and the gas produced in the melting gasifier is passed in the form of a reducing gas into the reducing shaft furnace.
German Patent 28 43 303 discloses a process for producing molten pig iron and reducing gas in a melting gasifier, in which the sponge iron particles are melted to molten pig iron or steel raw material and the heat and reducing gas necessary for melting are produced by the coal supplied and oxygen-containing gas blown in above the melt. In this process a relatively large amount of surplus or export gas is obtained and the energy consumption, i.e. the consumption of coal and oxygen is very high. If the export gas cannot be economically used, the pig iron production costs are very high. The chemical energy in the export gas is more than 50% of the energy supplied with the coal and the utilization of the reducing gas produced in the melting gasifier is max. 44%.
It is known from U.S. Pat. No. 4,225,340 to substantially free from carbon dioxide the top gas of a reducing shaft furnace and to use the thus prepared top gas for producing fresh reducing gas. It is supplied to the starting gas of a fossil fuel-processing gasifier, a gas reactor connected behind the latter and also the starting gas of the gas reactor either directly or via a gas heater. The starting gas of the gas reactor with the admixed, prepared top gas is supplied as reducing gas to the reducing shaft furnace. However, this known process is only used for producing sponge iron without using a melting gasifier.
A similar sponge iron production process is known from U.S. Pat. No. 4,260,412, in which top gas of a reducing shaft furnace prepared by carbon dioxide removal is used for producing fresh reducing gas.
The gas producer is a fluidized bed gasifier, to which are supplied the prepared top gas, coal and lime, as well as oxygen and optionally water vapour.
With the starting gas of said gasifier is admixed the prepared top gas both directly and via a gas heater, before it is passed into the reducing shaft furnace. This process also operates without a melting gasifier for the production of pig iron.
German Patent 34 38 487 discloses a process for producing pig iron, in which the top gas of the reducing shaft furnace is at least partly freed from the oxidizing constituents (CO.sub.2 and H.sub.2 O) and the thus prepared top gas is supplied to the melting gasifier. However, no economically satisfactory solution is found for the utilization of the energy rich top gas.
A complete gas recycling following the purification stage into the melting gasifier is only possible with coals having a volatile constituent content below 25% (waf). The heating of the top gas after purification by means of heat exchangers is subject to limits, which are prejudicial to an effective utilization.